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A. G. Enriquez and C. A. Friehe

Abstract

Aircraft measurements during the winter 1989 Shelf Mixed Layer Experiment (SMILE) and summer 1982 Coastal Ocean Dynamics Experiment (CODE) were used to characterize the spatial variation of the low-level wind and wind stress over the northern California shelf. The curl of the wind stress was calculated from directly measured turbulent stress components. The accuracy of the computed curl was estimated to be adequate to map the spatial structure. Wintertime measurements showed a concentration of large positive curl [over 1 Pa (100 km)−1] west of Point Arena, regardless of wind direction, due to the effects of the coastal topography on the wind fields. Results from summertime measurements showed a similar local maximum of positive curl west of Point Arena. Larger curl values [over 3.5 Pa (100 km)−1], however, were observed across a hydraulic jump propagating from Stewarts Point for highly supercritical marine boundary-layer flow.

A two-layer, vertically integrated numerical model of coastal upwelling was used to assess the relative importance of the stress curl to the stress-driven transport. The nonzero stress curl altered the thickness of the upper layer considerably after a day of integration, expanding the horizontal extent of upwelling offshore. The greatest effects were around areas of high positive curl, increasing coastal upwelling for downcoast winds and decreasing downwelling for upcoast winds. The effect of the stress curl, however, was attenuated near the coast as compared to the maximum possible deep water values. The validity of the numerical model was verified by comparison with an analytical solution of a simplified set of one-dimensional, frictionless equations of motion.

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C. E. Dorman, A. G. Enriquez, and C. A. Friehe

Abstract

The structure of the lower atmosphere over the northern California coastal ocean upwelling area was studied during the Shelf Mixed Layer Experiment in the winter of 1989. Surface data were collected at seven automated coastal stations and six buoys. Boundary layer soundings were made using balloons at the coast and a research aircraft over the ocean. The aircraft was also used to map the low-level (30 m) mean and flux fields over the 80 km × 120 km shelf area.

The wintertime coastal weather conditions were more variable than in summer and were observed to fit into three categories: strong northerly (downcoast) winds, strong southerly (upcoast) winds, and weak winds. The variability was caused by the passage of wintertime cyclones interspersed with periods of small pressure gradients. The strong wind cases had small diurnal variations, whereas the diurnal variations were large for the weak wind case.

The vertical structure of the coastal boundary layer was more uniform compared to that in summer, with weak or nonexistent temperature inversions. Winds below 600 m were not correlated with those above 1.5 km except during strong alongshore winds. The presence of a coastal mountain ridge suppresses low-level cross-shore flow. The horizontal structure over the ocean shelf measured by the low-level aircraft tracks showed an area of large positive wind stress curl [over 1 Pa (100 km)−1] west of Point Arena for both directions of the strong wind cases. This implies positive Ekman pumping of the shelf waters in this area regardless of wind direction.

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Robert C. Beardsley, Amelito G. Enriquez, Carl A. Friehe, and Carol A. Alessi

Abstract

An intercomparison between low-level aircraft measurements of wind and wind stress and buoy measurements of wind and estimated wind stress was made using data collected over the northern California shelf in the Shelf Mixed Layer Experiment (SMILE). Twenty-five buoy overflights were made with the NCAR King Air at a nominal altitude of 30 m over NOAA Data Buoy Center (NDBC) environmental buoys 46013 and 46014 between 13 February and 17 March 1989; meteorological conditions during this period were varied, with both up- and downcoast winds and variable stability. The buoy winds measured at 10 m were adjusted to the aircraft altitude using flux profile relations, and the surface fluxes and stability were estimated using both the TOGA COARE and bulk parameterizations.

The agreement between the King Air wind speed and direction measurements and the adjusted NDBC buoy wind speed and direction measurements was good. Average differences (aircraft − buoy) and standard deviations were 0.6 ± 0.8 m s−1 for wind speed and 0.0° ± 10.5° for direction (adjusted for buoy offset), independent of parameterization used.

The comparisons of aircraft and buoy wind stress components also showed good agreement, especially at larger values of the wind stress (>0.1 Pa) when the wind stress field appeared to be more spatially organized. For the east component, the average difference and standard deviation were 0.018 ± 0.029 Pa using TOGA COARE and −0.018 ± 0.027 Pa using . For the north component, the average difference and standard deviation are 0.003 ± 0.018 Pa using TOGA COARE and 0.003 ± 0.017 Pa using . These results support the idea that low-flying research aircraft like the King Air can be used to accurately map both the surface wind and the surface wind stress fields during even moderate wind conditions.

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